The present invention relates to a veneer lathe improved on the basis of a new technical idea.
In most conventional veneer lathes, driving force is exerted on the butt ends of a log by means of the chucks of spindles which grips the butt ends directly. In such a veneer lathe, since the radius of the chuck is too small as compared with the radius of a log, the gripped butt ends of the log cannot withstand the cutting resistance of a cutting knife and thereby the torsional breakage of the butt ends and the fructure of the log are liable to occur frequently. Therefore, most logs cannot be turned to a diameter as great as the diameter of the spindles.
In order to overcome above-mentioned disadvantages of conventional veneer lathes, the applicant of the present invention developed and proposed a new veneer lathe as disclosed in Japanese Patent Publication No. 56-16729, which comprises a rotary roller provided with a plurality of driving members each having a plurality of spikes, the driving members being arranged axially on the rotary roller at suitable intervals, the rotary roller being disposed practically in parallel with the cutting edge of a cutting knife at a position where the spikes can bite into the circumference of a log near the cutting edge during cutting operation; a driving mechanism for driving the rotary roller; and pressure members, such as fixed bars or rollers, disposed at least part of a plurality of interspaces formed between the driving members of the rotary roller.
In the above-mentioned new veneer lathe, since driving force is exerted on the periphery of a log near the cutting edge of the cutting knife through the driving members of the rotary roller, at least part of the driving force to be conventionally exerted by the spindles on the log can be omitted, the breakage of a log due to the concentration of stress on the chucked butt ends can be prevented, and the breakage of a log due to the clogging of a space between the periphery of the log and the pressure members with foreign matters such as the bark of logs and wood chips is prevented and hence almost every log can be turned down to about the diameter (usually, about 100 mm for logs of 1 m in length) of the spindles and thereby the yield of veneer sheets is improved remarkably.
Although the stripping of logs down to the diameter of about 100 mm for a log of 1 m in length was considered acceptable enough in times when logs of comparatively large outside diameters were readily available. However, it is not sufficient today when large-diameter logs are hardly available, much less in the future when the outside diameters of available logs will be reduced still further. Accordingly, further stripping of logs beyond the above mentioned level is desired as a matter of course.
Nevertheless, even in the above-mentioned new veneer lathe, it has been difficult to cut the log to a further reduced outside diameter thereof only by simply reducing the outside diameter of the spindles because various factors which will be described afterward forestall such a scheme. That is, the geometrical engagement between the periphery of a log and the driving members of the rotary roller decreases sharply as the outside diameter of the log decreases. On the other hand, the spikes of the driving members must be made to bite considerably deeply into the circumference of the log in order to secure the transmission of driving force necessary for turning operation. As a result the log is subjected to an increased pressure from the roller. Furthermore the deflection of a cylindrical log increases, as well known, in inverse proportion to the fourth power of the outside diameter thereof. More specifically, the deflection of the log increases sharply when the outside diameter of the log is reduced below 100 mm until finally the log is broken during turning operation or veneer sheets of irregular thickness unsuitable for use are produced. The existance of radial cracks extending from the core of a log, which are seen in the butt ends of ordinary logs, is a significant factor causing the log breakage when the outside diameter of the log is extremely small and is an additional impediment to turning a log to a further reduced outside diameter.